The present application relates generally to power distribution systems and, more particularly, to methods of operating power distribution systems using an enhanced partial differential protection scheme.
Known electrical distribution systems include a plurality of switchgear lineups including circuit breakers that are each coupled to one or more loads. The circuit breakers typically include a trip unit that controls the circuit breakers based upon sensed current flowing through the circuit breakers. More specifically, the trip unit causes current flowing through the circuit breaker to be interrupted if the current is outside of acceptable conditions.
Some known circuit breakers are programmed with one or more current thresholds (also known as “pickup” thresholds) that identify undesired current levels for the circuit breaker. If a fault draws current in excess of one or more current thresholds for a predetermined amount of time, for example, the trip unit typically activates the associated circuit breaker to stop current from flowing through the circuit breaker. However, in power distribution systems that include a plurality of circuit breakers, a typical arrangement uses a hierarchy of circuit breakers. Large circuit breakers (i.e., circuit breakers with a high current rating) are positioned closer to a power source than lower current feeder circuit breakers and feed the lower current feeder circuit breakers. Each feeder circuit breaker may feed a plurality of other circuit breakers, which connect to loads or other distribution equipment.
A fault may occur anywhere in the circuit breaker hierarchy. When a fault occurs, each circuit breaker that has the same fault current flowing through it may detect different amounts of fault current as a result of varying sensor sensitivities and/or tolerances. When the fault occurs, the circuit breaker closest to the fault should operate to stop current from flowing through the circuit breaker. If a circuit breaker higher in the hierarchy trips, multiple circuits or loads may unnecessarily lose service.
To accommodate for the varying tolerances and to ensure that multiple circuit breakers do not unnecessarily trip based on the same fault current, the current thresholds of at least some known circuit breakers are nested with each other to avoid overlapping fault current thresholds. In some other known systems, circuit breakers in a lower tier send coordination or blocking signals to higher tier circuit breakers upon detection of a fault current. The upper tier circuit breakers' operation is coordinated with the operation of the lower tier circuit breaker in response to the blocking signal.
In certain system topologies, circuit breakers known as ties, which connect distribution busses in the same tier of a system with multiple sources supplying multiple busses, cannot detect fault current direction. The trip unit in the tie does not know whether current is flowing through the tie from right to left or left to right. When a fault occurs the tie must send a blocking signal to upper tier devices on all connected sources. This results in the undesirable operation that all source devices are blocked when it would otherwise be desired that at least one of them not be blocked.
A protection scheme called bus differential protection is sometimes implemented, particularly in medium and high voltage systems, to improve upon protection and selective capability of hierarchical systems with one or more intermediary buses. In a system employing a bus differential protection scheme, current signals from all sources and loads must be available at a single circuit protective device. The circuit protection device that receives all of the current signals can identify if there is a fault within the differential zone. This scheme typically requires dedicated sensors at each circuit breaker in the system. The sensors are all coupled to the protective device providing the differential protection. The dedicated sensors generally require a relative high degree of accuracy and sensing quality, causing them to be relatively large. Bus differential protection is difficult to implement in low voltage systems because of the cost, performance issues during high-current faults, current transformer saturation issues, and the size of the additional sensors.
Another protection scheme used in some medium voltage systems is known as partial differential protection. Partial differential protection systems monitor only the source circuits and ties that can contribute fault current, but not the load circuits that may receive the fault current. The current flowing through each circuit breaker connected between a source and a protected bus is measured and provided to a single circuit protection device. The circuit protection device receiving the current data sums the current vectors to determine whether a fault is located outside its protection zone (i.e. on a different bus) or in its protection zone (i.e., on or below its distribution bus). Partial differential protection schemes require fewer current sensors than full bus differential protection schemes, because they do not monitor current through the load/feeder circuit breakers. Partial differential protection is generally cheaper and easier to implement than full bus differential protection. Unlike full bus differential protection schemes, partial differential schemes cannot identify the particular location of a fault within its protection zone (e.g., whether a fault is on the bus or downstream at one or more of the loads connected to the bus). However, partial differential protection schemes can identify when a fault is outside its protection zone, including on the bus itself and connected bus loads.